In recent years, the enlargement of the diameter of silicon wafers in use for semiconductor devices has advanced to ensure the number of chips per area effectively. Silicon single crystals having, for example, a diameter of 450 mm are desired for the next generation, but not currently mass-produced. It is unclear what type of wafers will be a major type.
Wafers having a diameter of 300 mm, currently being used, are however required strictly to have high quality: standard wafers need to have at least defect-free portions near wafer surfaces on which devices operate. Examples of major wafers to meet this requirement include epitaxial wafers, annealed wafers, defect-free (N-region) crystal polished wafers (PW).
Among them, the epitaxial wafer and annealed wafer have relatively wide margin for manufacture at the time of crystal growth even if defects are formed during the crystal growth, because defects near a surface layer are removed by formation of an epitaxial layer or annealing.
For the defect-free crystal PW, which is obtained by merely growing a defect-free crystal and polishing the crystal, growth conditions that enable a defect-free crystal to be grown need be achieved.
Defect-free crystals are obtained by holding the ratio V/G of a growth rate V to a temperature gradient G near the crystal growth interface at a constant value. These crystals are also obtained by controlling the value V/G so as to be constant in a crystal growth plane.
It is, however, very difficult to control the value V/G completely so as to be constant in the plane, so it is necessary to have the so-called margin for manufacture, which allows manufacture even when the value V/G slightly changes.
Patent Document 1 discloses a method of increasing the margin for manufacture: it is useful to rapidly cool a crystal. According to this method, a 7% margin for manufacture, which is sufficient for industrial manufacture, can be maintained.
Patent Document 2 discloses a technology of using a cooling cylinder and an auxiliary cooling cylinder, used as a rapid cooling means, in a CZ single crystal manufacturing apparatus. Patent Document 3 discloses a means for improving cooling capacity by improving the adhesiveness of an auxiliary cooling cylinder. It is obvious that rapid cooling of crystals with these technologies allows defect-free crystals to be obtained more easily.
Silicon single crystals having a diameter of 450 mm however have longer distance from the center than do those having a diameter of 200 mm or 300 mm, which are currently major, and hence difficulty in cooling its central portion. It is necessary for the 450-mm-diameter crystals to improve the cooling effect in comparison with the 200- or 300-mm-diameter crystals in order to raise the cooling rate at the central portion to the same level as the 200- or 300-mm-diameter crystals. The additional improvement of the cooling effect thus makes the internal stress of the crystals larger. The larger internal stress may cause problems such as crystal collapse during growth.
In view of these problems, Patent Document 4 discloses the inhibition of stress at the growth interface for prevention of crystal collapse. The conditions used therein, however, cannot be normally used because dislocations are generated in crystals before the crystal collapse. Moreover, Patent Document 5 is characterized by a thermal stress value of less than 40 MPa in a temperature range of 900° C. to 1100° C., but this condition is not sufficient.